The invention relates to oscillators, including varactor tuned oscillators (VCOs), operational in the millimeter wave range including frequencies greater than 30 GHz, and more particularly to circuitry for maintaining a near constant operating frequency of the oscillator over a large ambient temperature range to provide improved frequency versus temperature stability. The invention also provides improvements in further miniaturization and reduced cost.
Oscillators, including VCOs, are used extensively in military and commercial electronic equipment as local oscillators for transmitter sources. Frequency stability over a wide ambient temperature range is a basic requirement for high quality performance. In many applications, such as missile seekers of smart munitions, miniature size and low cost are concomittant requirements with frequency/temperature stability. A means to stabilize the operating frequency of oscillators in the millimeter wave range, including frequencies greater than 30 GHz, over a wide ambient temperature range was not heretofore realistic due to physical and electrical constraints in extrapolating conventional technology from the microwave range.
Conventional oscillator technology and stabilization techniques that have been used in the microwave frequency range up to 18 GHz become mechanically difficult to implement, costly and generally impractical when applied to the millimeter wave range, e.g., 30 to 100 GHz. This basically follows from the diminutive part size dictated by an extrapolation from conventional temperature compensated microwave oscillator technology. For example, mechanical arrangements with a differential expansion tuning element are conventionally used in the microwave bands to compensate for a change in oscillator frequency due to the dimensional change in oscillator cavity size with temperature. At X-band (8 to 12 GHz) the cross section of a typical rectangular waveguide cavity is 0.900 inches wide by 0.400 inches high. At millimeter wavelengths, e.g., 60 GHz, the cross section of a rectangular waveguide cavity would be only 0.148 inches wide by 0.074 inches high. These relatively small dimensions place severe if not impractical constraints on the size and fabrication tolerances needed for a differential expansion type frequency compensator. These constraints in conjunction with the precise mechanical adjustment needed for proper frequency compensation over a wide ambient temperature range increase cost and present substantial practical problems of reproducibility as a production oriented technique.
Another example of the limitation to the extension of conventional frequency compensated oscillator technology to the millimeter wave range is dielectric resonator oscillator (DRO) technology. This technology is near the limit of its effectiveness and practicality in the 20 GHz range. The upper frequency limit of commercial DROs is only 20 GHz after many years of DRO technology development. This is due to the small size and low O of a dielectric resonator when scaled for use at millimeter frequencies. The high dielectric constant of the resonator ceramic type material is another contributor to both the diminishing size of the resonator and its low O factor. The significant degradation in DRO performance at millimeter wavelengths is evidenced by reported results with experimental DROs at 26 GHz, "A 26 GHz Miniaturized MIC Transmitter/Receiver", E. Hagihara, H. Ogawa, N. Imai and M. Akaike, IEEE Trans. MTT, Vol. MTT-30, No. 3, March 1982, and 87 GHz, "Stabilization of a W-Band Microstrip Oscillator by a Dielectric Resonator", G. Morgan, Electronic Letters, June 24, 1982, Vol. 18, No. 13. The 26 GHz DRO had a frequency temperature stability of 1.4 ppm/.degree.C. over a temperature range limited to -5.degree. C. to +50.degree. C., and the 87 GHz DRO had a significantly degraded stability of 23 ppm/.degree.C. over a more restricted temperature range of +20.degree. C. to +60.degree. C. The constriction of temperature range with increasing frequency is an indicator of the loss of effectiveness of DRO technology in the millimeter wave range.
Reaction or transmission cavity type stabilization using low expansion material, e.g., invar, also has limited effectiveness in the millimeter range due to small cavity size and concomitant low Q factor, and restricted operating temperature range.
Conventional stabilization techniques have been limited to oscillators and have not been applied to broadband VCOs because of the difficulty in matching positive and negative reactance slope changes of an oscillator with that of a compensating structure over a wide frequency and temperature range. Frequency stabilization of a VCO by use of heater power to maintain a near constant VCO temperature has been used but is not a viable approach in systems with several VCOs and limited prime power, e.g., a missile seeker or a smart munition. Frequency compensation of a VCO by an external thermistor circuit has been reported, "Thermal Compensation of Varactor Tuned Oscillators", E. Levine, Microwaves and RF, August 1983, pp. 81-83, but compensation is only marginally effective (12 ppm/.degree.C.) over the limited temperature range of -5.degree. C. to +55.degree. C.
Problems and deficiencies of the prior art are summarized as follows:
Known frequency stabilization techniques are not adequate and therefore are not used for stabilization of microwave and millimeter wave VCOs over broad frequency and temperature ranges.
It is difficult if not impractical to extrapolate known microwave stabilization techniques such as dielectric resonators (DROs), cavity stabilizers or differential expansion compensators to the millimeter range for frequency stabilization of oscillators.
The use of an external heater to maintain a constant oscillator or VCO temperature is disadvantageous because of significant prime power consumption.
The size of oscillators stabilized by known techniques is large for applications requiring oscillators of miniature size. A distributed type circuit, i.e., waveguide, is inherently larger than the lumped element compensated circuit of the present invention.
The basis of known frequency stabilization techniques is to match positive and negative reactance slope changes of the oscillator over frequency and temperature with that of some form of a compensating element, e.g., differential expansion tuning element. Frequency stability with temperature has also been achieved by minimizing the reactance change of the oscillator circuit elements themselves over temperature, e.g., use of low expansion invar as a cavity material.
Known frequency compensation techniques address reactance change with temperature as an average effect for the total oscillator circuit. Known causes of frequency drift with temperature are the cavity embodiment, the temperature dependence of the active element, e.g., Gunn or Impatt diode, or FET, and in the case of a VCO, the temperature dependence of varactor capacitance. Known techniques treat all of these effects as a sum. In the present invention, lumped or printed element forms of the circuit embodiment used provide open and ready access to individual circuit elements, e.g., Gunn and varactor diodes, and each element can be individually compensated, thereby providing a more exact compensation.
Known frequency compensation of a microwave oscillator is waveguide form with a high dielectric constant (e.g., greater than 40) capacitor, "Simple Stabilizing Method for Solid State Oscillators", A. Kondo, T Ishii, and K. Shirahata, IEE Trans. in Microwave Theory and Techniques, November 1974, pp. 970-972, is difficult if not impractical to implement at millimeter wave frequencies due to its diminishing size with frequency. In the distributed type circuit described in Kondo et al, the compensating capacitor constitutes an additional circuit element. In the lumped element or printed circuit forms of the present invention, the compensating capacitor can be substituted for the oscillator resonator capacitor and thereby not increase the parts count. In addition, the circuit form of the present invention permits use of a low dielectric constant compensator material, e.g. less than 15, which is particularly desirable for higher frequencies. The compensating capacitor of the present invention can also be printed in-situ with the other elements of a printed circuit since specific and known substrate materials used for microwave printed circuits are also suitable for an appropriately sized compensating capacitor. The compensating capacitor described in Kondo et al cannot be printed due to its complex material composition and dissimilarity to substrate materials suitable for printed microwave and millimeter wave circuits.
The physical form of a distributed circuit, i.e., waveguide, results in a relatively larger circuit size than that of the lumped element circuit of the present invention. Larger size means that a larger temperature difference can exist between various parts of the circuit when a change in ambient temperature occurs. Incomplete frequency compensation occurs until a steady state condition is reached. In the lumped element embodiment of the present invention, a steady state condition is reached in a shorter time than in a distributed circuit because of the smaller size, e.g., 1/8 inch by 1/8 inch at 40 GHz. Hence, the compensate lumped element oscillator, including VCO, frequency stabilizes more rapidly than a compensated distributed type oscillator.